Catalyst preparation method

ABSTRACT

A method for preparing a catalyst comprising (i) preparing a calcined shaped calcium aluminate catalyst support, (ii) treating the calcined shaped calcium aluminate support with water, and then drying the support, (iii) impregnating the dried support with a solution containing one or more metal compounds and drying the impregnated support, (iv) calcining the dried impregnated support, to form metal oxide on the surface of the support and (v) optionally repeating steps (ii), (iii) and (iv) on the metal oxide coated support. The method provides an eggshell catalyst in which the metal oxide is concentrated in an outer layer on the support.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/318,273filed Jan. 17, 2012, which is a U.S. National Phase application of PCTInternational Application No. PCT/GB2010/050624, filed Apr. 15, 2010,and claims priority of British Patent Application No. 0907539.1, filedMay 1, 2009, the disclosures of which are incorporated herein byreference in their entireties for all purposes.

FIELD OF THE INVENTION

This invention relates to a method of preparing catalysts supported oncalcium aluminate.

Calcium aluminate-supported catalysts are used in a number of industrialprocesses, including methanation and steam reforming processes, such aspre-reforming, primary reforming and secondary reforming. In such casesthe catalytically-active metal is typically nickel, but other transitionmetals or precious metals, may also be used.

In methanation and steam reforming processes, pellets comprising nickeloxide on alumina or calcium aluminate are typically installed and thereduction of the nickel oxide to the active elemental nickel carried outin-situ.

U.S. Pat. No. 4,707,351 describes steam-reforming catalysts made of alow-silica calcium aluminate cement composition in a saddleconfiguration. The catalysts were prepared by mixing calcium aluminatewith water and polyvinylacetate, stamping shapes from the resultingmaterial, drying and calcining the saddles at up to 1400° C. beforeimpregnation with nickel nitrate. The impregnated saddles were furtherdried and calcined to generate the catalyst precursor. In this process,the calcium aluminate support was hydrated during the shaping processand then calcined to increase its strength and define the micromimetricproperties prior to impregnation with nickel nitrate.

Heretofore it has been seen as necessary to impregnate calcium aluminatesupports to achieve a uniform dispersion of the metal compound withinthe pellets such that upon calcination the metal oxide is uniformlydispersed within the pellet, thereby maximising the metal surface areaand hence catalyst activity.

BACKGROUND OF THE INVENTION

We have found that by re-hydrating the surface of the calcined shapedcalcium aluminate catalyst support and then drying it, the support, onceimpregnated with a metal compound provides an egg-shell catalystprecursor in which the metal oxide formed upon calcination isconcentrated as an outer surface layer on the support and is notuniformly distributed. Moreover, the properties of such catalysts areenhanced in comparison to the known catalysts.

Accordingly, the invention provides a method for preparing a catalystcomprising the steps of:

-   -   (i) preparing a calcined shaped calcium aluminate catalyst        support,    -   (ii) treating the calcined shaped calcium aluminate support with        water, and then drying the support,    -   (iii) impregnating the dried support with a solution containing        one or more metal compounds and drying the impregnated support,    -   (iv) calcining the dried impregnated support, to form metal        oxide on the surface of the support and    -   (v) optionally repeating steps (ii), (iii) and (iv) on the metal        oxide coated support.

The invention further provides an eggshell catalyst obtainable by themethod.

The invention further provides a process for the steam reforming ofhydrocarbons comprising the step of contacting a mixture of hydrocarbonand steam at elevated temperature and pressure with the eggshellcatalyst.

SUMMARY OF THE INVENTION

By the term “eggshell catalyst” we mean that the catalytically activemetal or metals are not uniformly distributed within the catalystsupport but are concentrated at the surface and therefore form a thinlayer, with the metal or metals being absent beneath this layer. Thethickness of the eggshell layer is preferably ≦1000 μm, more preferably≦800 μm, most preferably ≦300 μm.

The catalyst support is prepared from a calcium aluminate cement. By theterm calcium aluminate cement we include those hydraulic cementscontaining one or more calcium aluminate compounds of the formulanCaO.mAl₂O₃ where n and m are integers. Example of such calciumaluminate compounds include calcium monoaluminate (CaO.Al₂O₃),tricalcium aluminate (3CaO.Al₂O₃), penta calcium trialuminate(5CaO.3Al₂O₃), tricalcium penta aluminate (3CaO.5Al₂O₃), and dodecacalcium hepta aluminate (12CaO.7Al₂O₃). Some calcium aluminate cements,e.g. the so-called “high alumina” cements, may contain alumina inadmixture with, dissolved in, or combined with, such calcium aluminatecompounds. For example, a well known commercial high alumina cement hasa composition corresponding to about 18% calcium oxide, 79% alumina and3% water and other oxides. This material has a calcium:aluminium atomicratio of about 1:5, i.e. 2CaO.5Al₂O₃. Calcium aluminates are oftencontaminated with iron compounds, but these are not believed to bedetrimental to the present invention. Suitable cements include thecommercially available Ciment Fondu, and Secar 50, Secar 71, Secar 80available from Kemeos and CA-25, CA-14, CA-270 available from Almatis.

The support composition employed in the present invention preferably hasa calcium:aluminium atomic ratio within the range 1:3 to 1:12, morepreferably 1:3 to 1:10, most preferably 1:4 to 1:8. Where the calciumaluminate cement is a “high alumina” cement, no additional alumina maybe necessary but, in general, the support is desirably made from acalcium aluminate cement to which an additional amount of alumina, whichmay be in the form of a transition alumina, monohydrate or trihydrate,has been added.

To accelerate setting, an amount of lime (CaO), e.g. up to 15% by weightof the composition, may also be incorporated into the supportcomposition.

Hence the support will usually be a refractory composition consisting ofa calcined mixture of alumina, one or more of said calcium aluminatecompounds and optionally lime.

Other oxidic materials, e.g. titania, zirconia or lanthana, may bepresent in the calcium aluminate support composition. While silica mayin some cases be incorporated, for use as a steam reforming support, alow silica content, i.e. less than 1% by weight, preferably less than0.5% by weight, based on the weight of the oxidic material in thesupport composition is desirable, as silica has an appreciablevolatility under steam reforming conditions. The support compositionpreferably contains ≦25% by weight, more preferably ≦15% by weight, mostpreferably ≦10% by weight of oxidic material other than calciumaluminate and alumina.

The shaped catalyst support may be made by forming a calcium aluminatecement powder, optionally with additional alumina and/or lime, into thedesired shape, curing the cement and subsequently calcining the shapedsupport.

Processing aids, such as graphite and/or a metal stearate (e.g. Mg or Alstearate), may be incorporated into the composition prior to shaping:typically the proportion of graphite is 1 to 5% by weight of thecomposition. The amounts of metal stearate included may be in the range0.1 to 2.0% by weight.

A typical composition suitable for pellet formation comprises 30 to 70%by weight of a calcium aluminate cement (comprising 65 to 85% by weightof alumina and 15 to 35% by weight of CaO) mixed with 24 to 48% byweight of alumina, 0 to 15% by weight of lime, and 2 to 5% by weight ofgraphite.

The composition is desirably shaped into pellets using known techniques,but may also be prepared as extrudates or granules. The length, widthand height of such shaped units may be in the range 3-50 mm. The supportmay be in the form of saddles as described in the aforesaid U.S. Pat.No. 4,707,351 but preferably the support is pressed into pellets in theform of cylinders, which may have one or more through holes, for exampleas described in WO 2004/014549. More preferably the shaped support is inthe form of a cylindrical pellet having between 1 and 12 holes extendingthere-through, especially 3-10 holes of circular cross section, andoptionally between 2 and 20 flutes or lobes running along the length ofthe pellet. Suitable diameters for such pellets are in the range 4-40 mmand the aspect ratio (length/diameter) is preferably ≦2. A particularlypreferred shape is a highly domed cylindrical pellet in the form of acylinder having a length C and diameter D, which has one or more holesextending therethrough, wherein the cylinder has domed ends of lengths Aand B, such that (A+B+C)/D is in the range 0.50 to 2.00, and (A+B)/C isin the range 0.40 to 5.00. Such shapes are described in co-pending WO2010/029323 A1 and WO 2010/029324 A1. C is preferably in the range 1 to25 mm and D is preferably in the range 4 to 40 mm.

After shaping, the cement in the shaped catalyst support should be curedand the support dried, typically at under 200° C., and then calcined.Curing of the calcium aluminate cement may take place before or during adrying step, e.g. by spraying or immersing the shaped catalyst supportwith water prior to drying or by heating the shaped catalyst supportunder conditions of controlled relative humidity prior to volatilisingresidual water. Calcination is typically carried out by heating theshaped units to between 500 and 1400° C. in air for between 1 and 16hours. The catalyst support strength increases, while the porosity andsurface area decrease, as the calcination temperature increases. Hencethe support calcination should be effected at sufficient temperature toobtain the required mechanical strength but should not be so high thatthe surface area and porosity are unduly reduced.

The shaped calcined catalyst support preferably has a total surfacearea, as measured by nitrogen absorption, of 0.5 to 40, particularly 1to 15, m²g⁻¹. and a pore volume of 0.1 to 0.3 cm³·g⁻¹, as determined bymercury porosimetry.

Prior to the final calcination, the support may be “alkalised” byimpregnation with a solution of an alkali such as potassium hydroxide.This serves to minimise lay down of carbon on the catalyst during steamreforming resulting from high temperature cracking of hydrocarbons andfrom the reaction of carbon oxides with hydrogen. Alkali oxide, e.g.potash, levels of up to about 5% wt on the calcined support may be used.

In the present invention, prior to impregnation with a metal compound,the calcined shaped calcium aluminate support is subjected to are-hydration step by treating the support with water. The water isdesirably free of salts and is preferably demineralised water ordeionised water. Small amounts of organic base or ammonia may be addedto the water. The treatment of the shaped calcium aluminate support withwater may be by immersion or spraying with water at ambient or elevatedtemperature. The re-hydration step should be performed for a periodsufficient to react the surface of the calcined shaped calcium aluminatesupport with water. In a preferred embodiment, the treatment with wateris by immersing the shaped support in water for a period of between 1and 120 minutes at a temperature in the range 10-95° C. Using water at atemperature of ≦30° C. has been found to give thinner metal oxidelayers, which is advantageous, particularly for small catalysts orcatalysts with through-holes. The re-hydration step may be performed atatmospheric or elevated pressure.

Following the treatment of the surface of the calcined support withwater, the support is dried to remove the water, preferably to removethe physisorbed water and not the chemisorbed water. Thus the supportdrying is preferably performed at a temperature in the range 25-250° C.,more preferably in the range 50-150° C. at atmospheric or reducedpressure. Drying times may be in the range 1-24 hours depending upon thewater content.

Without wishing to be bound by theory, it is believed that there-hydration and drying steps change the surface chemistry of thecalcium aluminate support. Evidence indicates that the pores in thesurface of the support are not blocked; rather that the surface basicityof the support is affected so that upon impregnation, insolublecompounds of the metal impregnate precipitate at/near the surface of thesupport thereby generating the eggshell catalyst.

The re-hydrated and dried catalyst support is then impregnated with asolution comprising one or more soluble metal compounds. Theimpregnation solution preferably comprises one or more transitionmetals, preferably one or more selected from the group consisting ofchromium, manganese, nickel, cobalt, iron, copper and zinc. Morepreferably the impregnation solution comprises one or more of nickel,cobalt, iron or copper, most preferably nickel.

Aqueous impregnation solutions are particularly suitable. Theimpregnation solution preferably comprises one or more acidic compounds,i.e. compounds that dissolve in water to give acidic solutions (i.e. theimpregnation solution desirably has a pH<7.0). Suitable acidic metalcompounds include metal nitrate, metal acetate metal citrate and metaloxalate. Where the impregnated metal is nickel, the metal compound usedto impregnate the support is preferably nickel nitrate or nickelacetate.

The concentration of metal in the impregnating solution is desirably inthe range 100-300 g metal/litre.

Impregnation may be performed at ambient or elevated temperature and atatmospheric or elevated pressure using known techniques, includingimmersion of the re-hydrated and dried catalyst support in ametal-containing solution or by so-called “incipient wetness”impregnation where the volume of solution used equates approximately tothe pore volume of the support material. Impregnation of the metalcompound at ambient temperature (i.e. 10 to 25° C.), and at atmosphericpressure (about 1 bar abs) may be used, however it has been found thatby impregnating the support at temperatures in the range 50-90° C.,improved control over the thickness of the eggshell layer may beobtained. For example, the eggshell layer thickness on pelletedmaterials may be ≦800 μm at 20-30° C., but impregnation at 50-90° C. canproduce thicknesses of ≦300 μm.

Following impregnation, the impregnated support is dried and calcined.Drying conditions are preferably the same as those used following there-hydration step. The calcination step to convert the impregnated metalcompound to its corresponding metal oxide is preferably performed in airat a temperature in the range 250-850° C. An advantage of the presentinvention, by virtue of the lower metal content and the increased metalconcentration at the surface of the catalysts, is that the amount ofnitrogen oxides evolved during calcination of metal nitrate-basedprecursors can be reduced compared with current catalyst materials.

The catalytic metal content of the resulting catalyst may be determinedby a number of factors such as the metal content of the solution and theimpregnation conditions. Steam reforming catalysts formed byimpregnation typically have a NiO content in the range 10-35% wt.Precipitated pre-reforming catalysts can have NiO contents of 40-80% wtor more. Methanation catalysts typically have a NiO content in theregion of 30-35% wt. In the present invention, because the catalyticmetal oxide is concentrated at the surface of the support it is possibleto achieve improved catalyst activity with reduced metal loadings. Thishas clear commercial benefits. The catalytic metal oxide content of thecalcined catalyst is preferably in the range 2-25% wt, preferably 4-15%wt. Thus one impregnation may be sufficient to generate the desiredcatalyst. However if desired, the re-hydration, drying and impregnationsteps may be repeated until the metal oxide content of the calcinedmaterial is above 2.5% wt, preferably above 5% wt, more preferably above7.5% wt, most preferably above 10% wt. Multiple impregnations may beperformed using the same or different catalytically active metal. Inorder to maintain the eggshell catalyst, the metal oxide containingsupport should be re-hydrated, dried and calcined prior to each metalimpregnation.

The specific surface area of the catalytic metal is suitably in therange 0.1 to 50 m²/g of catalyst. Within this range, larger areas arepreferred for reactions under 600° C.

One or more promoter compounds may be impregnated into the dried supportand/or the metal oxide coated support. Hence one or more promotercompounds may be included in the metal impregnating solution or thepromoter may be added subsequently by a separate impregnation. Thepromoter may be confined to the eggshell layer or may be distributedthroughout the catalyst support. Promoters include precious metals suchas platinum, palladium, iridium, ruthenium, rhodium and gold. Lanthanidemetals such as lanthanum and cerium may also be included as promoters.Water-soluble salts, particularly nitrates, may be used as sources ofthe metal promoters. More than one promoter may be present andadditional alkali may also be added. The amount of promoter metal willtypically be in the range 0.1-5% wt on the calcined catalyst material.

The catalysts may be provided and used in their oxidic form. E.g. oxidicCo catalysts may be used for oxidation reactions.

Where the catalyst comprises a reducible metal such as Cu, Ni, Co or Fe,the calcined product may be provided in its oxidic form and reduction ofthe metal oxide, if required, to form elemental metal carried outin-situ, i.e. in the reactor in which the catalyst is to be used, with ahydrogen-containing gas. Known reduction techniques may be used.

Alternatively, the oxidic catalyst may be reduced ex-situ and then theelemental metal coated with a thin passivating layer of oxide using anoxygen containing gas. In this way the catalyst may be transportedsafely to the user, and the reduction time to generate the activecatalyst and quantity of hydrogen used during the subsequent activation,reduced. This has clear benefits for the user. Therefore in oneembodiment, the method for preparing the catalyst further comprises thesteps of reducing a reducible metal oxide to elemental form with ahydrogen-containing gas mixture and subsequently passivating the surfaceof the elemental metal with an oxygen-containing gas. Oxygen and carbondioxide gases may be used for example as described in U.S. Pat. No.4,090,980.

The eggshell catalysts prepared according to the invention may be usedin steam reforming processes such as primary steam reforming, secondaryreforming of a primary reformed gas mixture and pre-reforming. Thecatalysts may also be used for methanation reactions, hydrogenationreactions and, in oxidic unreduced form, for the decomposition ofhypochlorite in aqueous solutions.

In steam reforming, a hydrocarbon, typically a methane-containing gassuch as natural gas or naphtha is reacted with steam and/or, whereappropriate, carbon dioxide, over a catalytically active material, oftencomprising nickel, to produce a gas containing hydrogen and carbonoxides. The hydrogen producing reactions are:

CH₄+H₂O

CO+3H₂

“CH₂”+H₂O→CO+2H₂

(“CH2” represents hydrocarbons higher than methane, for example normallygaseous hydrocarbons and normally liquid hydrocarbons boiling at up to200° C.). The analogous reactions with carbon dioxide can be carried outseparately or with the steam reaction.

CH₄+CO₂→2CO+2H₂

“CH₂”+CO₂→2CO+H₂

These reactions are strongly endothermic and the process is especiallysuitable when they are carried out with external heating as in tubularsteam reforming. Alternatively the heat can be supplied by heating thereactants and passing steam over the catalyst in an adiabatic bed or ina hybrid process in which oxygen is a reactant, so that heat evolved inoxidation is absorbed by the endothermic reactions. The hybrid processcan be applied to the product of the tubular or adiabatic process thatis, in “secondary reforming”, or to fresh feedstock (“catalytic partialoxidation” or “autothermal reforming”). Commonly these reactions areaccompanied by the water-gas shift reaction:

CO+H₂O

CO₂+H₂

If the starting hydrocarbon is “CH₂” and the temperature is relativelylow, the methanation reaction (exothermic) may also occur.

CO+3H₂→CH₄+H₂O

CO₂+4H₂→CH₄+2H₂O

However, the steam reforming process is operated preferably in netendothermic conditions and the hydrogen containing gas produced containsat least 30% v/v of hydrogen on a dry basis. Preferably it contains lessthan 30, especially less than 10, % v/v of methane on a dry basis. Forthe production of hydrogen-containing synthesis gas, the outlettemperature is preferably at least 600° C. to ensure low methanecontent. While the temperature is generally in the range 750-900° C. formaking synthesis gas for ammonia or methanol production, it may be ashigh as 1100° C. for the production of metallurgical reducing gas, or aslow as 700° C. for the production of town gas. For the hybrid processusing oxygen, the temperature may be as high as 1300° C. in the hottestpart of the catalyst bed.

In pre-reforming, a hydrocarbon/steam mixture is subjected to a step ofadiabatic low temperature steam reforming. In such a process, thehydrocarbon/steam mixture is heated, typically to a temperature in therange 400-650° C., and then passed adiabatically through a fixed bed ofa suitable particulate catalyst, usually a catalyst having a high nickelcontent, for example above 40% by weight. The catalysts may be simplecylinders or a multiholed, lobed shape. Pre-reforming catalysts aretypically provided in a pre-reduced and passivated form, although oxidiccatalyst may also be installed. During such an adiabatic low temperaturereforming step, any hydrocarbons higher than methane react with steam onthe catalyst surface to give a mixture of methane, carbon oxides andhydrogen. The use of such an adiabatic reforming step, commonly termedpre-reforming, is desirable to ensure that the feed to the steamreformer contains no hydrocarbons higher than methane and also containsa significant amount of hydrogen. This is desirable in order to minimisethe risk of carbon formation on the catalyst in the downstream steamreformer. The pressure in steam reforming processes is typically in therange 1-50 bar abs. but pressures up to 120 bar abs. are proposed. Anexcess of steam and/or carbon dioxide is normally used, especially inthe range 1.5 to 6. for example 2.5 to 5, mols of steam or carbondioxide per gram atom of carbon in the starting hydrocarbon.

Where the catalyst is to be used for methanation, in order to remove lowconcentrations of CO and CO₂ (0.1-0.5% vol) from a hydrogen-containinggas, the hydrogen-containing gas is typically passed through aparticulate fixed bed of a nickel-containing catalyst at a temperaturein the range 230-450° C. and pressures up to about 50 bar abs or higherup to about 250 bar abs. Unlike steam reforming the catalyst arepreferably simple cylindrical pellets without through holes, althoughsuch pellets may be used if desired. Typical pellet diameters are in therange 2.5-6 mm, with lengths in the same range. The catalysts may beprovided in oxidic form or pre-reduced and passivated form.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is also illustrated by reference to the Examples and FIGS.1-3.

FIG. 1 depicts an image of a cylindrical catalyst pellet cut in two toshow an eggshell layer of catalyst prepared according to the presentinvention;

FIG. 2 depicts an image of a similar catalyst pellet prepared accordingto the prior art, and

FIG. 3 depicts an electron-probe micro-analyser (EMPA) image of a crosssection of a lobed 4-hole cylindrical catalyst pellet with an eggshelllayer of catalyst prepared according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Examples Example 1 Preparation ofa Catalyst Support

a) Calcium aluminate cement was blended with alumina trihydrate and limeto obtain a mixture with a Ca:Al ratio of 10:43. Graphite (4 wt %) wasadded, and the resulting mixture pelleted using a hydraulic tablettingmachine to give cylinders of diameter 3.3 mm and length 3.3 mm. Thepellets were subjected to water-curing and calcination to obtain acalcined shaped support with the following properties.

BET (Nitrogen): 5.7 m²/g

Pore volume: 0.28 cm³/g

Density: 1.66 g/cc

b) The method of Example 1(a) was repeated to produce pellets ofdiameter 5.4 mm and length 3.0 mm.

c) The method of Example 1(a) was repeated except that calcium aluminatecement was blended with alumina trihydrate to obtain a mixture with aCa:Al ratio of 10:74.

Example 2 Preparation of a Ni Catalyst

a) Re-hydration. The shaped calcined calcium aluminate support fromExample 1(b) was treated with water by immersing the pellets inde-mineralised water at 30° C. for 40 minutes. The pellets were removedand dried at 110° C. for 16 hours.

b) Incorporation of Ni. The catalyst support pellets were then immersedin a solution of nickel nitrate in de-mineralised water (200 g Ni/litre)for 5 minutes at 25° C. The impregnated pellets were then removed andallowed to drain for 10 minutes and dried at 110° C. for 6 hours. Thedried impregnated pellets were then heated at 100° C./hour to 650° C.and then held at 650° C. for 4 hours to convert the nickel nitrate tonickel oxide. The re-hydrating, drying, impregnating, drying andcalcining procedure was repeated on the nickel oxide containing pelletsa further two times. The Ni was concentrated in a thin layer around theedge of the catalyst pellet as shown in FIG. 1. The thickness of theeggshell layer is about 800 μm.

The procedure was repeated on the support of Example 1(a), using thesame water treatment conditions but carrying out the Ni impregnationeach time for 5 minutes at 70° C. instead of 25° C. The weight increaseafter each calcination was measured on 20 pellets and an average taken.The results were as follows;

Weight of Weight of oxidic NiO NiO loading Impregnation Support (g)catalyst (g) weight (g) (% wt) 1 0.045 0.0456 0.0009 1.974 2 0.0450.0475 0.0028 5.895 3 0.045 0.0485 0.0038 7.835

The final catalyst properties were;

BET (Nitrogen): 43.6 m²/g

Pore volume: 0.17 cm³/g

Density: 1.84 g/cc

This material was termed catalyst 2A.

The procedure applied to the support of Example 1(a) was repeated on thesupport of Example 1(c), using the same water treatment and impregnationconditions. An egg-shell catalyst material was produced.

In comparison, the shaped calcined calcium aluminate support fromExample 1(b), but without the above re-hydration step, was immersed in asolution of nickel nitrate in de-mineralised water (200 g Ni/litre) for5 minutes at 25° C. The impregnated pellets were then removed andallowed to drain for 10 minutes and dried at 110° C. for 6 hours. Thedried impregnated pellets were then heated at 100° C./hour to 650° C.and then held at 650° C. for 4 hours to convert the nickel nitrate tonickel oxide. The impregnating, drying and calcining procedure wasrepeated on the nickel oxide containing pellets a further two times. TheNi was distributed throughout the catalyst pellet as seen in FIG. 2.

As a further comparison, the shaped calcined calcium aluminate supportfrom Example 1(a), again without the re-hydration step, was immersed ina solution of nickel nitrate in de-mineralised water (200 g Ni/litre)for 5 minutes at 70° C. The impregnated pellets were then removed andallowed to drain for 10 minutes and dried at 110° C. for 6 hours. Thedried impregnated pellets were then heated at 100° C./hour to 650° C.and then held at 650° C. for 6 hours to convert the nickel nitrate tonickel oxide. The impregnating, drying and calcining procedure wasrepeated on the nickel oxide containing pellets a further two times. TheNiO content of this material after the final calcination was about 16.5wt %. This comparative material was termed catalyst 2B.

Example 3 Testing

The catalysts 2A and 2B were tested in a laboratory scale steam reformerwith a reformer tube internal diameter of 1-inch. The catalysts werediluted with fused alumina chips (sieve fraction 3.35 mm-4.74 mm) andreduced using 50 vol % H₂ in N₂ at 480° C. for 2 hours. After catalystreduction, catalyst performance was assessed over the temperature range480° C. to 750° C. The feed gas was natural gas mixed with steam at asteam:carbon ratio of 3.0:1. The exit gas composition was analysed byinfra-red and gas chromatography.

The results were as follows;

Catalyst 2A 2B Ethane Ethane Improvement by conversion (%) conversion(%) egg-shelling 600° C. 77 54 41% 540° C. 66 43 53% 480° C. 44 29 50%670° C. 83 66 26% 750° C. 88 76 16% 670° C. 82 63 31%

The ethane conversion is better for the eggshell catalyst 2A across thetemperature range.

Example 4 Preparation of Catalysts

a) Catalyst Support. Calcium aluminate cement was blended with aluminatrihydrate and lime to obtain a mixture with a Ca:Al ratio of 10:43.Graphite (4 wt %) was added, and the resulting mixture pelleted using ahydraulic tabletting machine to give cylinders of diameter 5.4 mm andlength 5.4 mm. The pellets were subjected to water-curing andcalcination to obtain a calcined shaped support with the same propertiesas example 1(a).

b) Re-hydration. The shaped calcined calcium aluminate support wastreated with water by immersing the pellets in de-mineralised water at30° C. for 40 minutes. The catalyst support pellets were removed fromthe water and dried at 110° C. for 16 hours.

c) Catalyst Preparation. The catalyst support pellets were immersed in asolution of metal nitrate in de-mineralised water as detailed below for5 minutes at 25° C.

Metal nitrate Concentration (g of metal/100 ml) Co 10 Cu 10 Ni 10

The impregnated pellets were then removed and allowed to drain for 10minutes and dried at 110° C. for 12 hours.

The pellets were analysed by optical microscopy and in each case it wasfound that the metal compound was concentrated around the edge of thepellet, i.e. that an eggshell material had been formed.

In comparison, the shaped calcined calcium aluminate support, withoutthe re-hydration step (b), was immersed in the solution of metal nitratein de-mineralised water as detailed above for 5 minutes at 25° C. Theimpregnated pellets were then removed and allowed to drain for 10minutes and dried at 110° C. for 12 hours. The pellets were analysed byoptical microscopy and in each case the metal was distributed throughoutthe catalyst.

Example 5 Preparation of Catalyst with Ni Acetate

a) Catalyst Support. Calcium aluminate cement was blended with aluminatrihydrate and lime to obtain a mixture with a Ca:Al ratio of 10:43.Graphite (4 wt %) was added, and the resulting mixture pelleted using ahydraulic tabletting machine to give cylinders of diameter 5.4 mm andlength 5.4 mm. The pellets were subjected to water-curing andcalcination to obtain a calcined shaped support with the same propertiesas example 1(a).

b) Re-hydration. The shaped calcined calcium aluminate support from wastreated with water by immersing the pellets in de-mineralised water at30° C. for 40 minutes. The pellets were removed and dried at 110° C. for16 hours.

c) Catalyst preparation. The pellets were then immersed in a solution ofNi-acetate (2 g/100 ml) in de-mineralised water for 5 minutes at 25° C.

The impregnated pellets were then removed and allowed to drain for 5minutes and dried at 110° C. for 4 hours.

The pellet was analysed by optical microscopy and it was found that thenickel was concentrated around the edge of the pellet.

Example 6 Shaped Catalyst Support with Through Holes

A 4-holed, 4-lobed catalyst calcined calcium aluminate catalyst supportwas prepared according to the method of Example 1. The catalyst supportwas re-hydrated and impregnated three times at 25° C. with nickelnitrate according to the method of Example 2. Electron-probemicro-analysis (EPMA) of a cross section of the resulting dried andcalcined catalyst pellet showed a thin layer of nickel oxide (lighterarea) around the outside of the pellet and around the circumference ofeach of 30 the through holes. The EPMA image is depicted in FIG. 3.

1. An eggshell catalyst comprising a layer of nickel oxide of thickness≦1000 μm on the surface of a calcined, shaped calcium aluminate cementsupport.
 2. An eggshell catalyst according to claim 1 wherein thecalcium aluminate support comprises calcium aluminate cement powder andat least one of alumina and lime.
 3. An eggshell catalyst according toclaim 1 further comprising an alkali metal oxide.
 4. An eggshellcatalyst according to claim 1 wherein the support is in the form of ashaped pellet or extrudate.
 5. An eggshell catalyst according to claim 4wherein the support is in the form of a cylindrical pellet havingbetween 1 and 12 holes extending there-through.
 6. An eggshell catalystaccording to claim 5 wherein the support is in the form of a cylindricalpellet having between 1 and 12 holes extending there-through and between2 and 20 flutes or lobes.
 7. An eggshell catalyst according to claim 6wherein the support has 4 holes extending there-though and 4 lobes. 8.An eggshell catalyst according to claim 1 wherein nickel oxide contentof the catalyst is in the range 2-25% wt.
 9. A process for the steamreforming of a hydrocarbon comprising the step of contacting a mixtureof hydrocarbon and steam with a catalyst according to claim
 1. 10. Aprocess according to claim 9 wherein the steam reforming is selectedfrom primary steam reforming, and secondary reforming of a primaryreformed gas mixture.
 11. A process according to claim 9 wherein thehydrocarbon comprises methane or naphtha.
 12. A process according toclaim 9 wherein the steam reforming is performed with an outlettemperature in the range 700-1100° C.
 13. A process according to claim 9performed at a pressure up to 120 bar abs.
 14. A process according toclaim 9 wherein the steam is present in the mixture of hydrocarbon andsteam in the range of 1.5 to 6 moles of steam per gram atom of carbon inthe hydrocarbon.
 15. A process for the methanation of a hydrogen gasstream gas stream containing CO and CO₂ in amounts in the range 0.1-0.5%vol comprising the step of contacting the hydrogen gas stream at atemperature in the range 230-450° C. with a catalyst according toclaim
 1. 16. A process according to claim 15 performed at a pressure upto 250 bar abs.